Physical approaches and perspectives
Quantum networks extend from trusted node devices built on weak-pulse qkd systems to more advanced entanglement-based scenarios including quantum repeaters. The goal in terms of quantum communication is motivated by increasing both the distance of QKD and the complexity of these quantum networks- architectures are no longer constrained to be P2P. The extension to entangled systems also has the benifit of providing a means of wiring up quantum computing and simulation systems, which can either be compact, small-scale, systems in one lab, or in distributed schemes, and connecting different types of quantum processors. The foundations of these architectures relies on the distribution and control of entanglement across complex quantum networks and central to realising this is the most fascinating quantum phenomenon, teleportation. The development of complex quantum networks provide one of the most significant challenges in experimental quantum physics today. By definition this is highly multi-disciplinary and requires hybrid approaches on both a conceptual and technological level.
Trusted-Node Networks
Since the SECOQC QKD network demonstration in 2008 illustrated the concept of trusted-node QKD [1], the concept has been widely exploited. In any QKD scheme the communicating parties must be in secure locations, therefore, these can also be used to realise switching stations between multiple P2P systems, provided the operators at these nodes can be trusted. This field is primarily dominated by fibre systems though it does open up the possibility for satellite systems to connect to a larger and more complex quantum network. This is an important interim step (before quantum repeaters) as point-to-point quantum key distribution schemes approach their distance limits. These ideas have gone on to be demonstrated in networks in Switzerland, Japan, South Africa and Canada and many trusted node networks are running conntinuously in the locations on a research and commercial level.
European groups working in this field include: R. Alleaume (SeQureNet, FR), J. Capmany (Valencia, ES), N. Gisin and H. Zbinden (Geneva, CH), V. Martin (Madrid, ES), A. Poppe (AIT, AT), A. Shields (TREL, UK), P. Grangier (Paris, FR), G. Ribordy (IDQ, CH).
Quantum Networks and Teleportation
Trusted-node networks provide one solution for extending quantum communication distances and complexity. However, to move towards fully quantum-secure networks, and for the distribution of quantum resources in general, will require entanglement based schemes. To realise this will require quantum repeaters and although the development of these, and the quantum memories necessary for their operation, progresses, there remain significant challenges for the distribution of entanglement through complex fibre optic networks. For example, the synchronisation and stabilisation of these networks and the the high-fidelity Bell state measurements (joint measuremnets between two systems, two photons) necessary for teleportation and entanglement swapping, which are at the heart of all quantum repeater protocols, remain largely unstudied. Similarly for satellite systems where much of the Bell state measurements, teleportation and entanglement swapping needs to adapt from stationary ground based systems to moving, satellite, targets. There are several scenarios possible such as satellite to ground, or low orbiting platforms as well as ground to (trusted) satellite schemes. Field trials will be essential to understanding their practical limits. Furthermore, understanding how to characterise and quantify these increasingly complex systems is an ongoing problem that needs to go beyond standard approaches of quantum state and process tomography, especially if the security of the system is to be assured in a distributed network architecture.
European groups working in this field include: J. Rarity (Bristol, UK), S. Tanzilli (Nice, FR), R. T. Thew and N. Gisin (Geneva, CH), R. Ursin and A. Zeilinger (Vienna, AT), H. Weinfurter (Munich, DE).
Quantum Repeaters
In classical communication, information is transferred by encoding, modulating the intensity of, light fields. The amplitude of these modulations are detected by photodetectors, transformed into electrical current pulses, amplified by electronics, and sent to computers, phones, etc. This transformation of light into electrical signals forms a classical light-matter interface. In quantum information processing, this simple approach is inadequate as it destroys the quantum aspect. Quantum communication requires a coherent storage interface -- a quantum memory. Quantum memories are central to the concept of quantum repeaters. Quantum repeaters work by breaking large distances up into smaller ones where entanglement can be distributed and stored in quantum memories. Once all of these smaller links are entangled, Bell state measurements can be used to join them together, thus increasing the communication link distance. These quantum memories can also be thought of as small quantum processors and hence the idea of using similar the techniques learned from quantum repeaters for connecting the nodes of a quantum computer or simulator. There are a significant number of proposals for realising quantum repeaters ranging from atomic ensembles (cold and hot gases and solid state systems) and linear optics- perhaps the simpler and more advanced approach -to atom and ion approaches that could take advantage of deterministic entanglement swapping operations. Other approaches based on NV centres in diamonds and quantum dots have been proposed as well as hybrid schemes that combine coherent states and individual quantum systems. More information about quantum memories has already been presented and a detailed review of ensemble approaches using linear optics and discussions on several others can be found here [2].
European groups working in this field include: M. Afzelius and N. Gisin (Geneva, CH), J. Laurat and E. Giacobino (CNRS-Paris, FR), J. Rarity (Bristol, UK), E. Polzik (Copenhagen, DK), H. de Riedmatten (ICFO, ES), J. Schmiedmayer (Vienna, AT), I. Walmsley and J. Nunn (Oxford, UK), H. Weinfurter and G. Rempe (Munich, DE).
State of the art
Trusted node QKD systems have shown systems capable of fully automated operation, including self-compensation for environmental influences on the fibre link. The demonstrations have involved one-time pad encrypted telephone communication, secure (AES encryption protected) video-conferencing and rerouting experiments, highlighting basic mechanisms for quantum network functionality. In the SEQOQC network The average link length was between 20 and 30 km, the longest link 83 km and similar distances are in daily use now in several continuously operational networks around the world. Recently, experiments have addressed passive optical network(PON) implementations for QKD, covering 20 channels in the telecom band [3]. Teleportation experiments in the real world have been demonstrated in the Swiss fibre optic network (3x2 km) [4] as well as free-space transmission of teleported states in China (97 km) [5] and the Canary Islands (144 km) [6]. The first marks an important first step towards fibre-based quantum repeaters and the later for satellite systems. The synchronisation of independent sources for entanglement swapping has been realised using CW [7] and fs pulsed systems [8]. These results of these experiments highlight the two extremes of operation, in terms of photon bandwidth, for such experiments. Quantum repeaters represent one of the most rapidly evolving areas of activity in the field and progress is largely linked to the work on quantum memories and interfaces that have already been discussed. Several proof-of-principle repeater links have been demonstrated in atomic ensemble systems [9, 10] and with single atoms [11, 12] and ions [13] as well as demonstration of systems entangling single trapped ions/atoms/quantum-dots and single photons [14-16] as well as for entangling solid state quantum memories [17]. Distances of up to 300 m and coherence times of several 100 micro s [18] have been shown. Behind all of these experiments is an increased activity in more applied aspects of quantum communication related to the synchronisation and stabilisation of distributed quantum networks involving a wide range of different quantum technologies.
Challenges
There is no clear leader for quantum networks and long distance quantum communication with dedicated programs in place across Europe and in the USA, Canada, Japan, Australia and China. In the next 5-10 years we should see fibre optic systems that can beat the direct-transmission QKD distance limitation of around 300-400 km. Initially, quantum repeaters that can function over 1-10 km will provide the building blocks for longer transmission systems- it is these building blocks that provide a scalable route towards pan-European and even global scale quantum communication. These distances will obviously need to be extended further, but not necessarily by much, n.b. classical communication links are of the order of 50-100 km between amplification stages. One of the important aspect for quantum repeaters is the scaling of multiple quantum repeater links. Scalable quantum repeater systems will ensure that the concatenation of multiple links will extend quantum communication distances beyond this fundamental (loss-based) limit and away from the P2P network topologies. Effort in the next few years should be focused on engineering the sources, interfaces and detectors specifically adapted to long distance transmission and working in unison- long coherence lengths, and high fidelity Bell-State measurements. Challenges and directions of future work are thus similar to those already mentioned for these different technologies and while many aspects have been realised, all need to be improved and demonstrated in the one systems. Some of the key challenges are:
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Field demonstrations of multiplexed trusted node QKD systems running autonomously with Mbps secure key rates;
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Field demonstrations of quantum relays, exploiting quantum teleportation and entanglement swapping, over tens of km of with high fidelity (90%) Bell-State measurements;
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Explore hybrid approaches for quantum communcation networks that combine concepts from both discrete and CV regimes for improved performance;
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Incorporate deterministic strategies for sources, storage and entanglement swapping;
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Improve input/output efficiencies and coupling to fibre optic channels for diverse quantum memories suitable for quantum repeaters;
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Demonstrate coupling, via an optical quantum channel, between different quantum processing nodes.
Key references
[1] M. Peev et al., New J. Phys. 11, 075001 (2009)
[2] N. Sangouard, C. Simon, H. de Riedmatten, N. Gisin, Rev. Mod. Phys. 83, 33 (2011)
[3] J. Mora et al., Opt. Exp. 20, 16358 (2012)
[4] O. Landry et al., J. Opt. Soc. Am. B 24, 398 (2007)
[5] J. Yin et al., Nature 488, 185 (2012)
[6] X. S.Ma et al., Nature, doi:10.1038/nature11472
[7] M. Halder et al., Nature Physics, 3, 692 (2007)
[8] R. Kaltenbaek et al., Phys. Rev. A, 79, 040302(R) (2009)
[9] C.W. Chou et al., Science, 316, 1316 (2007)
[10] Z-S Yuan et al., Nature 454, 1098 (2008)
[11] J. Hofmann et al., Science 337, 72 (2012)
[12] S. Ritter et al., Nature 484, 195 (2012)
[13] D. N. Matsukevich et al., Phys. Rev. Lett. 100, 150404 (2008)
[14] B. B. Blinov et al., Nature, 428, 153 (2004)
[15] J. Volz et al., Phys. Rev. Lett. 96, 030404 (2006)
[16] R. M. Stevenson et al., Phys. Rev. Lett. 101, 170501 (2008)
[17] I. Usmani et al., Nature Photonics 6, 234 (2012)
[18] W. Rosenfeld et al., Phys. Rev. Lett. 101, 260403 (2008)
